The current multilayer bond pad structure for tape automated bonding comprises three different metal layers: aluminum, titanium-tungsten alloy, and gold. The aluminum layer is formed on the semiconductor substrate. The gold layer is the seed film for a subsequent Au bump plating process. The TiW alloy is the intended barrier layer between Al and Au to prevent the formation of the well-known "purple plague". The thicknesses of the various layers are: Al - 10,000 .ANG., TiW - 3,000 .ANG., and Au - 2,000 .ANG.. Typically, the ratio of Ti:W is about 10 at % Ti and 90 at % W.
At the present time, the sputter deposition of TiW and Au layers is done sequentially in one pumpdown to minimize contamination from the ambient. The problem observed with this multilayer structure is that the pure TiW layer, which served as a barrier layer, has been found to be ineffective, allowing Au and Al to intermix during a 450.degree. C./30 sec inner-lead bond (ILB) TAB operation. This intermixing is generally not favorable due to the formation of Au--Al intermetallics which are brittle in nature and sometimes highly resistive. During a thermal cycle, stress would be generated at the Au, Al, and intermetallics interfaces which would likely lead to microcracks. Upon further cycling, these microcracks could propagate to weaken the metallization/bump interface and eventually lead to failure. In plastic packages, Au--Al intermetallics and void formation have been the major cause of voltage overload (V.sub.OL) failures at high temperatures.
Interdiffusion problems in TAB structures similar to that described above are not new and have been described in the prior art by others. For example, R. S. Nowicki et al in Thin Solid Films, Vol. 53, pp.195-205 (1978) found that TiW used in an Al/TiW/Au metallization system exhibits marginal diffusion characteristics when deposited in a relatively pure state. They reported that an improvement was observed when TiW was sputtered in a gaseous mixture comprising nitrogen and argon. It appeared that optimum barrier properties were obtained when TiW was deposited in a relatively impure state, i.e., filled with nitride. However, gold does not stick well to a TiW-nitride surface.
It is desirable to have an effective barrier layer for tape automated bonding, to which gold will readily adhere.